The least expensive way I know of to get really long telephoto focal lengths would be to try and find an older Nikon CP series camera such as the CP950 or CP990 which are still sometimes available on ebay, etc.
With a CP950/990 or even a CP4500 you can attach to nearly any spotting scope. Sometimes you can find an older model Meade ETX-90 for under $200 and probably find a CP990 or CP950 for under $200. With this combination you can shoot at focal lengths up to 6000mm.
Here is a CP990 image taken through my Meade ETX-90 at 5989mm: